As described, for example, in U.S. Pat. Nos. 2,297,691 and 2,357,809, a conventional electrophotographic process comprises the steps of: uniformly charging a photoconductive insulating layer; selectively exposing the charged layer to light to dissipate the charges on the surface in the illuminated regions, to thereby form a latent electrostatical image; allowing a colored and charge-bearing fine powder, called a toner, to adhere to the latent image, to thereby make the latent image visible (development step); transferring the visible toner image to a receiving material such as paper (transfer step); and then permanently fixing the toner image to the receiving material by heating, pressing or some other suitable fixing means (fixing step).
The toner, therefore, should have all the functions required for not only the development step but the transfer and fixing steps.
In general, toners in a mechanical developing apparatus suffer mechanical friction when they receive shearing and impact forces caused by the mechanical operation of the apparatus, and the toners deteriorate when several thousand to several ten thousand copies are produced. Such a toner deterioration may be avoided by the use of a tough resin which has a high enough a molecular weight to be able to withstand the mechanical friction. However, resins of this kind generally have high softening points, so that fixing of toners employing such resins by a non-contact fixing method such as oven fixing or radiant fixing using infrared radiation cannot be sufficiently conducted because of poor thermal efficiency. Further, in the case of heated roller fixing which is a contact fixing technique extensively used because of its good thermal efficiency, there is the drawback that the temperature of the heated roller must be increased in order to attain sufficient fixing and the thus elevated roller temperature leads to a deterioration of the fixing apparatus, a curling of paper, an increase in the energy consumption, etc. In addition, if such resins are used for producing toners, production efficiency is considerably lowered since the pulverizability of such resins is poor. Because of these drawbacks, binder resins whose polymerization degrees and softening points are too high cannot be used.
The heated roller fixing method involves exceedingly good thermal efficiency because a heated roller is brought into contact with a receiving sheet so that the toner image on the receiving sheet is pressed by the heated roller surface. Although this fixing method is widely used at fixing speeds ranging from low to high due to its good thermal efficiency, an offset phenomenon is apt to occur where part of the toner adheres to the heated roller surface during contact of the heated roller with the toner image and is then transferred to a receiving paper or other receiving sheet. In order to avoid this phenomenon, rollers having surface layers made of a material with excellent release properties, such as a fluoroplastic, are employed and, in addition, a release agent such as a silicone oil or the like is coated on the roller surfaces. However, such a fixing apparatus in which a silicone oil or another release agent is coated on the roller surfaces is not preferred in that not only does the apparatus necessarily have an increased enlarged size and become more costly, but also the resulting complicated structure is prone to be a cause of problems.
As described in JP-B-57-493, JP-A-50-44836, and JP-A-57-37353, the offset phenomenon is less apt to occur if one uses a resin which has been made asymmetric and crosslinked. (The terms "JP-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined published Japanese patent application", respectively.) However, this expedient has failed to improve fixing properties.
In general, the lowest fixing temperature for a toner is between the cold offset-disappearing temperature and the hot offset-occurring temperature and, hence, the usable temperature range is from the lowest fixing temperature to the hot offset-occurring temperature. Therefore, by lowering the lowest fixing temperature as much as possible and by increasing the hot offset-occurring temperature as much as possible, the usable fixing temperatures can be lowered and the usable temperature range can be increased at the same time, thereby attaining energy saving, high speed fixing and prevention of paper curling.
For such reasons, there is a need for a resin or toner which always has good fixing properties and offset resistance.
In the case where a styrene-based binder resin is used to meet the above requirements, it is known that paraffin wax, a low-molecular polyolefin or the like can be added as an anti-offset agent, as described in JP-A-49-65232, JP-A-50-28840, and JP-A-50-81342. This technique, however, is disadvantageous because it has been ascertained that such an additive must be incorporated in a relatively large amount in order to produce the desired effect and the large additive amount results in accelerated deterioration of the developer.
Polyester resins originally possess good fixing properties, so that polyester resin based toners can be satisfactorily fixed even by non-contact fixing methods as described in U.S. Pat. No. 3,590,000. However, it is difficult to fix such toners by heated roller fixing because the offset phenomenon is apt to occur. Although polyester resins whose offset resistance has been improved by using polybasic carboxylic acids have been proposed in JP-A-50-44836, JP-A-57-37353, and JP-A-57-109875, the offset resistance of some of such polyester resins is still insufficient for practical use, and the other polyester resins have attained sufficient offset resistance at the expense of the low temperature fixing properties originally possessed by the polyester resins and further have had a problem that they show extremely poor pulverizability when formulated into toners.
Toners having smaller particle diameters are being developed for the purpose of obtaining higher resolution in electrophotography. However, the problem which remains unsolved is how such small particle diameter toners can be produced efficiently at low cost.
In the case of producing toners using a polyester resin having an acid value or hydroxyl value particularly suitable for improved dispersibility of colorants, such toners have been limited in composition, as compared with toners employing a styrene-acrylic resin, if they are required to have a sufficient charge amount and also to have good environmental stability of fixed images. Studies have, therefore, been made on the combined use of polyester resins and styrene-acrylic resins, and the following have, for example, been proposed: to merely blend a polyester resin with a styrene-acrylic resin as described in JP-A-49-6931, JP-A-54-114245, JP-A-57-70523, and JP-A-2-161464; to form chemical bonds between a polyester resin and a styrene resin as described in JP-A-56-116043; to copolymerize a vinyl monomer with an unsaturated polyester as described in JP-A-57-60339, JP-A-63-279265, JP-A-1-156759, and JP-A-2-5073; to copolymerize a vinyl monomer with a polyester resin having a (meth)acryloyl group as described in JP-A-59-45453; to copolymerize a reactive polyester with a vinyl monomer in the presence of a polyester resin as described in JP-A-2-29664; and to form a block copolymer by bonding a polyester resin and a vinyl resin through an ester bond as described in JP-A-2-881. However, since the compatibility between polyester resins and styrene-acrylic resins is poor, binder resins obtained by mere mechanical blending of these two different kinds of resins are disadvantageous in that, depending on to the proportion of each resin, toners employing such binder resins may give fixed images that cause ground sheet fouling. Further, the copolymerization of a vinyl monomer with a reactive polyester is limited with respect to the copolymerization used ratio in order to prevent gelation.